[release-20.0] Fix flaky tests that use vtcombo (#16178)

[vitess-bot]

Description

This is a backport of #16178

[vitess-bot]

Review Checklist

Hello reviewers! 👋 Please follow this checklist when reviewing this Pull Request.

General

• Ensure that the Pull Request has a descriptive title.
• Ensure there is a link to an issue (except for internal cleanup and flaky test fixes), new features should have an RFC that documents use cases and test cases.

Tests

• Bug fixes should have at least one unit or end-to-end test, enhancement and new features should have a sufficient number of tests.

Documentation

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• There should be some code comments as to why things are implemented the way they are.
• There should be a comment at the top of each new or modified test to explain what the test does.

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Backward compatibility

• Protobuf changes should be wire-compatible.
• Changes to _vt tables and RPCs need to be backward compatible.
• RPC changes should be compatible with vitess-operator
• If a flag is removed, then it should also be removed from vitess-operator and arewefastyet, if used there.
• vtctl command output order should be stable and awk-able.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.57%. Comparing base (6c591bd) to head (5edebcc).

Additional details and impacted files

@@               Coverage Diff                @@
##           release-20.0   #16213      +/-   ##
================================================
+ Coverage         68.53%   68.57%   +0.03%     
================================================
  Files              1541     1541              
  Lines            197733   197743      +10     
================================================
+ Hits             135526   135601      +75     
+ Misses            62207    62142      -65     

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